Pointers in C++

1. Pointers in C++ Topics to cover: • Overview of Pointers • Pointer Declaration • Pointer Assignment • Pointer Arithmetic • Relations Between Pointers and Arrays • Pointers and Strings Computer Skills2 for Scientific Colleges

2. Overview of Pointers • A Pointer in C++ is variable whose value is a memory address. • With pointers many memory locations can be referenced. • Some data structures use pointers (e.g. linked list, tree). • The * and & operators - & operator is the address operator - * operator is the dereferencing operator. It is used in pointers declaration Computer Skills2 for Scientific Colleges

3. Pointer Declaration • Pointers are declared as follows: <type> * variable_name ; e.g. int * xPtr; // xPtr is a pointer to data of type integer char * cPtr; //cPtr is a pointer to data of type character void * yPtr; // yPtr is a generic pointer, // represents any type Computer Skills2 for Scientific Colleges

4. Pointer Assignment • Assignment can be applied on pointers of the same type • If not the same type, a cast operator must be used • Exception: pointer to void does not need casting to convert a pointer to void type • void pointers cannot be dereferenced • Example int *xPtr, *yPtr; int x = 5; … xPtr = & x; // xPtr now points to address of x yPtr = xPtr; // now yPtr and xPtr point to x Computer Skills2 for Scientific Colleges

5. Pointer Arithmetic • Increment / decrement pointers ( ++ or -- ) • Add / subtract an integer to/from a pointer ( + or += , - or -= ) • Pointers may be subtracted from each other • Pointer arithmetic is meaningless unless performed on an array Computer Skills2 for Scientific Colleges

6. Pointer Arithmetic (Cont.) • Example Consider an integer array of 5 elements on a machine using 4 bytes for integers. 1000 1004 1008 1012 1016 Pointer variable vPtr • vPtr pointes to first element V[0] (location 1000) i.e. vPtr = 1000 • vPtr +=2; sets vPtr to 1008 i.e. vPtr points to V[2] Computer Skills2 for Scientific Colleges

7. Pointer Arithmetic (Cont.) • Subtracting pointers - Returns the number of elements between two addresses e.g. if v is an array and vPtr1 = v[0]; vPtr2 = v[2]; then vPtr2 – vPtr1 = 2 (i.e. 2 addresses) Computer Skills2 for Scientific Colleges

8. Relations Between Pointers and Arrays • Arrays and pointers are closely related. - Array name is like constant pointer - Pointers can do array subscribing operations - If we declare an array A[4] and a pointer aPtr  aPtr is equal to A aPtr == A  aPtr is equal to the address of the first element of A aPtr == & A[0] Computer Skills2 for Scientific Colleges

9. Relations Between Pointers and Arrays (Cont.) • Accessing array elements with pointers: - Element A[i] can be accessed by *(aPtr+i)  This is called pointer/offset notation - Array itself can use pointer arithmetic  A[3] is same as *(A+3) - Pointers can be subscripted (i.e. pointer/subscript notation)  aPtr [3] is same as A[3] Computer Skills2 for Scientific Colleges

10. Examples on Pointers //File: swap.cpp //A program to call a function to swap two numbers using reference parameters  #include <iostream.h> void swap(int *, int *); // This is swap's prototype void main() { int x = 5, y = 7; swap(&x , &y); // calling swap with reference parameters cout << "\n x is now "<< x << " and y is now " << y << '\n'; } // swap function is defined here using dereferencing operator ‘*’ void swap(int *a, int *b) { int temp; temp = *a; *a = *b; *b = temp; } Computer Skills2 for Scientific Colleges

11. Examples on Pointers (Cont.) //File: pointers.cpp //A program to test pointers and references #include <iostream.h> void main ( ) { int intVar = 10; int *intPtr; // intPtr is a pointer intPtr = & intVar; cout << "\nLocation of intVar: " << & intVar; cout << "\nContents of intVar: " << intVar; cout << "\nLocation of intPtr: " << & intPtr; cout << "\nContents of intPtr: " << intPtr; cout << "\nThe value that intPtr points to: " << * intPtr; } Computer Skills2 for Scientific Colleges

12. Pointers and Strings • Pointers can be used to declare strings and with string functions (see next lecture slides) Computer Skills2 for Scientific Colleges